gh-137508: Improve list.remove/index error message

Doc/library/doctest.rst

@@ -471,10 +471,10 @@ Simple example::
    >>> [1, 2, 3].remove(42)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
-   ValueError: list.remove(x): x not in list
+   ValueError: 42 not in list
 
-That doctest succeeds if :exc:`ValueError` is raised, with the ``list.remove(x):
-x not in list`` detail as shown.
+That doctest succeeds if :exc:`ValueError` is raised, with the
+``42 not in list`` detail as shown.
 
 The expected output for an exception must start with a traceback header, which
 may be either of the following two lines, indented the same as the first line of

Lib/multiprocessing/shared_memory.py

@@ -539,6 +539,6 @@ def index(self, value):
             if value == entry:
                 return position
         else:
-            raise ValueError("ShareableList.index(x): x not in list")
+            raise ValueError(f"{value} not in list")
 
     __class_getitem__ = classmethod(types.GenericAlias)

Misc/NEWS.d/next/Core_and_Builtins/2025-08-07-10-22-21.gh-issue-137508.8IyyiL.rst

@@ -0,0 +1,2 @@
+:func:`list.remove` and :func:`list.index` now raise with more informative
+error messages.

Misc/NEWS.d/next/Library/2025-08-07-10-23-40.gh-issue-137508.wDo29Q.rst

@@ -0,0 +1,2 @@
+:meth:`multiprocessing.shared_memory.ShareableList` now rasies with a more
+informative error message.

Objects/listobject.c

@@ -3305,7 +3305,7 @@ list_index_impl(PyListObject *self, PyObject *value, Py_ssize_t start,
         else if (cmp < 0)
             return NULL;
     }
-    PyErr_SetString(PyExc_ValueError, "list.index(x): x not in list");
+    PyErr_Format(PyExc_ValueError, "%R not in list", value);
     return NULL;
 }
 
@@ -3375,7 +3375,7 @@ list_remove_impl(PyListObject *self, PyObject *value)
         else if (cmp < 0)
             return NULL;
     }
-    PyErr_SetString(PyExc_ValueError, "list.remove(x): x not in list");
+    PyErr_Format(PyExc_ValueError, "%R not in list", value);
     return NULL;
 }